Earnings Per Share (details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended									6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Net income	$ (8,233)	$ 28,147	$ (10,879)	$ 1,227	$ 40,284	$ 28,766	$ (8,527)	$ 1,985	$ 40,820	$ 41,511	$ 31,532	$ 30,632	$ 58,779	$ 63,044	$ 72,013
Average common shares outstanding - basic	26,987,000		26,847,000								26,962,000	26,813,000
Additional shares for stock based compensation plans	0		0								51,000	89,000
Average common shares outstanding - diluted	26,987,000		26,847,000								27,013,000	26,902,000
Earnings per share of common stock - basic	$ (0.31)	$ 1.05	$ (0.41)	$ 0.05	$ 1.50	$ 1.08	$ (0.32)	$ 0.07	$ 1.53	$ 1.55	$ 1.17	$ 1.14	$ 2.19	$ 2.36	$ 2.71
Earnings per share of common stock - diluted	$ (0.31)	$ 1.04	$ (0.41)	$ 0.05	$ 1.50	$ 1.07	$ (0.32)	$ 0.07	$ 1.53	$ 1.54	$ 1.17	$ 1.14	$ 2.18	$ 2.36	$ 2.70
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Antidilutive Shares Excluded from Net Income Per Diluted Common Share Calculation	80,000		107,000								26,000	0